It is common practice to use a scintillator in combination with an image sensor to capture x-ray images. In such a setup, the image sensor is placed behind the scintillator. However, by laws of nature, the scintillator can only absorb a certain fraction of all x-ray photons that enter the scintillator's surface. As a consequence, it is important for best detector performance and image quality to best utilize the information carried by each x-ray photon that is absorbed in the scintillator. In the scintillation process, the energy of an x-ray photon is transferred to a large number of secondary, visible, photons which can be detected by the image sensor. Because of noise generated in the image sensor, it is important to construct the scintillator so as to maximize the number of secondary photons that reach and can be detected by the image sensor. This will result in an x-ray detector with good signal-to-noise ratio.
It is a general requirement in x-ray imaging to achieve the best possible image quality, often interpreted as the balance between high resolution (sharpness) and high signal-to-noise ratio. These two image requirements are typically contradictory so that high resolution often is accompanied by reduced signal-to-noise ratio and vice versa.
Various techniques have been proposed for the fabrication of a structured scintillator, which is based on a micromechanical structure such as a structured array or matrix of pores or elongated trenches filled with scintillating material that would provide light guiding of secondary photons to an underlying imaging sensor. These techniques are all restricted in one or several aspects: either too large lateral dimensions (cutting, dicing), problems of forming a well-defined narrow wall (laser ablation), cross talk between adjacent pixels (columnar growth technique) or a lengthy processing time (valid for most of these techniques).
Deposition of a reflective coating in the bottom of the micromechanical structure such as pores has thus been suggested to improve light guiding and reduce cross talk, but designing and producing a feasible and efficient solution is generally not an easy task considering the manufacturing process, the narrow geometry of the micromechanical structure and materials involved.
U.S. Pat. No. 6,744,052 generally concerns the basic design of a structured scintillator, and also introduces a quite satisfactory solution for providing light guiding of secondary photons based on an embedded reflective coating in the scintillator.
U.S. Pat. No. 6,344,649 relates to a scintillator having a plurality of scintillator elements laid out in an array. The scintillator elements are fabricated from polycrystalline ceramic scintillator material or single crystal scintillation material. To increase the spatial resolution and the signal strength, the gaps between the scintillator elements are filled with a reflective material.
U.S. Pat. No. 6,177,236 relates to a pixelated scintillation layer in which high aspect ratio columns of scintillating material are formed. Wells may be formed in a body, and filled with scintillation material dispersed in a solvent/binder. A reflective coating may be deposited over the surface of the wells, e.g. by aluminum evaporation or electrochemical deposition.
U.S. Pat. No. 5,519,227 relates to a structured scintillation screen, where a pixelated structure having well-defined spatial geometries and depths is micro-machined using laser ablation. Following laser processing of the substrate, the ‘pixels’ are surrounded with an interstitial material having a refractive index lower than that of the substrate to allow each pixel to function as an individual optic waveguide.
EP 0,534,683 relates to a radiation imager comprising an array of scintillator elements optically coupled to a photodetector array. Interstitial wall members separate adjoining scintillator elements. A solution for reflection of light photons back into the scintillator elements is provided by means of a dual-layer reflective structure that comprises a primary dielectric layer of lower optical index to reflect light photons at the interface of the scintillator element and the dielectric layer, and a supplementary optically reflective layer to allow those light photons that strike the dielectric layer with an angle of incidence greater than the critical angle and enter the dielectric layer to reflect off of the supplementary optically reflective layer.
WO 2012/004703 relates to a scintillator having an array of so-called scintillator dixels that are separated by spacers. The spacers include a reflective material that facilitates directing light produced by a dixel to a corresponding light-sensing region of a photo-sensor array.
WO 2014/109691 relates to an x-ray scintillator comprising a pore matrix having a plurality of pores formed in a substrate. Each of the pores is at least partially covered with a multi-layered coating comprising at least a reflective layer and a protective layer. The at least partially coated pores are filled with scintillating material for absorbing x-ray photons to produce secondary photons, preferably with a wavelength in the visible range. The reflective layer of the multi-layered coating is arranged between the scintillating material and the substrate for reflecting the secondary photons, and the protective layer of the multi-layered coating is arranged between the reflective layer and the scintillating material for protecting the reflective layer while allowing reflection of the secondary photons by the reflective layer.
However, there is still a general demand for even more efficient solutions, especially for the purpose of increasing the signal-to-noise ratio without reducing the image resolution (sharpness).